1. Field of the Invention
The invention relates to the process of fabricating integrated circuits. More specifically, the invention relates to a method and an apparatus for performing target-image-based proximity correction for advanced photomasks during fabrication of an integrated circuit.
2. Related Art
Recent advances in integrated circuit technology have largely been accomplished by decreasing the feature size of circuit elements on a semiconductor chip. As the feature size of these circuit elements continues to decrease, circuit designers are forced to deal with problems that arise as a consequence of the optical lithography process that is typically used to manufacture integrated circuits. This optical lithography process begins with the formation of a photoresist layer on the surface of a semiconductor wafer. A mask composed of opaque regions, which are generally formed of chrome, and light-transmissive clear regions, which are generally formed of quartz, is then positioned over this photoresist layer. (Note that the term “mask” as used in this specification is meant to include the term “reticle.”) Light is then shone on the mask from a visible light source, an ultraviolet light source, or more generally some type of electromagnetic radiation source together with suitably adapted masks and lithography equipment.
This image is reduced and focused through an optical system containing a number of lenses, filters, and mirrors. The light passes through the clear regions of the mask and exposes the underlying photoresist layer. At the same time, opaque regions of the mask block the light leaving underlying portions of the photoresist layer unexposed.
The exposed photoresist layer is then developed, through chemical removal of either the exposed or non-exposed regions of the photoresist layer. The end result is a semiconductor wafer with a photoresist layer having a desired pattern. This pattern can then be used for etching underlying regions of the wafer.
As integration densities continue to increase, it is becoming necessary to use phase-shifters to define more and more features within a layout. In addition to the mask containing these phase-shifters, a separate trim mask is used to selectively expose portions of the photoresist to correct (trim) the exposed areas of the photoresist to more accurately form the desired features of the integrated circuit. This is commonly known as a double exposure alternating aperture phase shift mask (AAPSM).
One problem that occurs during the optical lithography process is corner rounding. Corner rounding is caused by optical effects such as light being diffracted around the corner, and by uneven etching of the photoresist at the corner. Outside corners tend to cause the exposed image to be rounded inward, whereas inside corners tend to cause the exposed image to be rounded outward. An optical proximity correction (OPC) process adds corrections or serifs to the corners in the masks so that the exposed image comes closer to agreeing with the target image. As a convenience, the term OPC is used in the most general sense to encompass any and all types of proximity corrections performed as part of the design and lithography process. In any given design flow, the user may have an option to control which of the different types of proximity correction the OPC software will apply.
For conventional simple photomasks such as binary masks, the mask image corresponds well to the intended target wafer image. For example, an intended corner in the wafer appears to be a corner as well on the mask. Hence, the corresponding OPC strategies have mostly been focused on the mask features. However, in advanced photomasks such as double exposure AAPSM, the mask images do not directly correspond to the desired wafer image. An OPC process that targets the mask image or feature may not be effective as for a simple photomask as seen in the example below.
FIG. 1A illustrates a phase shift mask 102 for creating a feature on an integrated circuit. Phase shift mask 102 includes 0-degree phase shifter 104 and 180-degree phase shifter 106. During operation, electromagnetic radiation is passed through the 0-degree phase shifter 104 and 180-degree phase shifter 106. Optical interference caused by the difference in phase of the shifters causes the underlying photoresist between the two shifters to be left unexposed to help form a printed feature on an integrated circuit.
FIG. 1B illustrates a binary or trim mask used in conjunction with the phase shift mask for creating a feature on an integrated circuit. Binary mask 108 includes trim feature 110. Trim feature 110 can be formed from an opaque material such as chrome. During a second exposure of the photoresist using binary mask 108, the printed feature on the integrated circuit is further defined. The remaining areas of the mask 108 may include phase shifting features and/or attenuated material as appropriate.
FIG. 1C illustrates a desired feature 120 (lighter stipple and rectangular solid outline) and a printed feature 118 (darker stipple and curved solid outline) created by using the phase shift mask 102 and the binary mask 108 to expose a wafer. Desired feature 120 represents the desired feature to be printed on the integrated circuit, while printed feature 118 represents what is actually printed on the integrated circuit. The outlines of 0-degree phases shifter 104, 180-degree phase shifter 106, and trim feature 110 are shown for reference. Printed feature 118 includes several areas that differ from desired feature 120. In line end area pullback and rounding of the end of printed feature 118 caused by diffraction effects and resist effects is visible. Area 112 shows bulges in printed feature 118 caused by corner effects of phase shifters 104 and 106, and trim feature 110. Area 116 shows portions of printed feature 118 that are too narrow, possibly caused by edge diffraction effects. An optical proximity correction (OPC) process is therefore used in an attempt to make printed feature 118 more closely match desired feature 120 as described below in conjunction with FIGS. 2A-D.
FIG. 2A illustrates edges 202 on phase shifters 104 and 106 and edges 204 on trim feature 110 used to create a printed feature on an integrated circuit (both shown with thicker solid lines). Note that neither phase shifters 104 and 106 nor trim feature 110 individually describes the desired printed feature. The desired printed feature is evident only from the combination of exposing the photoresist through phase shifters 104 and 106, and trim feature 110.
FIG. 2B illustrates dissection points and evaluation points on phase shifter edges 202 and trim feature edges 204. During a traditional, mask image-based OPC process, edges 202 and 204 are dissected as illustrated at 206 and evaluation points are selected as illustrated at 208. Note that only some dissection points and evaluation points are numbered for simplicity. Short horizontal lines indicate the dissection points, while an “X” indicates evaluation points. During the OPC process each segmented edge based on the dissection points will be biased in or out according to computations made at the evaluation point to help the final image more closely approximate the target layout.
FIG. 2C illustrates the results of optical proximity correction on phase shifters 104 and 106, and trim feature 110. The OPC process follows shifter corner-to-corner minimum spacing rules at location 210. Shifter corner-to-corner minimum spacing rules are mask making requirements, and are also used sometimes to prevent certain potential printability problems. According to the mask images, areas 212 are corners and hence are corrected using tolerances and thresholds specified for corners. Hammerhead 216 is used to alleviate line end shortening problems. Similar to shifter corner-to-corner minimum spacing rules, trim corner minimum width rules apply at area 214.
FIG. 2D illustrates a printed feature 118 created from the corrected phase shifters 104 and 106, and from trim feature 110. Outlines for phase shifters 104 and 106 and trim feature 110 are shown for reference. Note that while the line ends and line edges are closer to desired feature 120, there is still a bulge at 218 due to insufficient compensation for the corner rounding effects of both masks.
Hence, what is needed is a method and an apparatus for performing optical proximity correction while minimizing the problems described above.